1. Field of the Invention
This invention relates to the migration of a molten zone through a solid body of semiconductor material by thermal gradient zone melting.
2. Description of the Prior Art
W. G. Pfann described in "Zone Melting", John Wiley and Sons, Inc., New York (1966), a thermal gradient zone melting process to produce various desirable material configurations in a body of semiconductor material. The process had previously been disclosed in his issued U.S. Pat. No. 2,813,048, based on his application filed June 24, 1954. In both instances, cavities are generally formed in the surface of the body and a piece of wire of the metal to be migrated is disposed on the cavity. However, the resulting structures were not desirable for semiconductor usage.
M. Blumenfeld, in U.S. Pat. No. 3,897,277, teaches alloying aluminum to the surface of the body of silicon semiconductor material in an attempt to maintain the registry of the pattern of metal deposits to be migrated. However, problems of precise registry of the metal still plague one's attempt to obtain the precision necessary to obtain an array of deep diodes suitable for making x-ray imaging devices.
Recently, Thomas R. Anthony and Harvey E. Cline, discovered that employing selective chemical etching of the surface and preferred crystallographic orientation of the surface and molten zones enabled one to employ thermal gradient zone melting processing to assist in making semiconductor devices commercially feasible. The improved process resulted in a large savings in energy required to process semiconductor materials and increased yields. For a further teaching of the improved process, one is directed to their teachings in their recently granted U.S. Pat. No. 3,904,442, and copending patent application Ser. No. 519,913 filed Nov. 1, 1974, now U.S. Pat. No. 3,979,230 assigned to the assignee of the present invention and incorporated therein by reference.
To practice thermal gradient zone melting on a commercial basis, one should make the process as simple as possible. Consequently, John Boah in his copending U.S. Pat. application Ser. No. 578,807 filed May 19, 1975, and now U.S. Pat. No. 4,001,047, describes the use of radiant energy as a source for the migration or movement of a molten zone through a solid body. This application is assigned to the assignee of the present invention and is incorporated herein by reference. The process and apparatus arrangement taught by Boah is very practical for the manufacture of many devices. In the manufacture of certain semiconductor devices, the maintenance of a temperature gradient precisely in a direction perpendicular to a major surface of the semiconductor body being treated is desirable. Maintenance of such a temperature gradient and the elimination of lateral or transverse temperature gradients ensures the precise orientation of junctions within the devices and maximization of manufacturing yields.
One source of radiant energy which is presently being employed in practicing temperature gradient zone melting is quartz lamps having tungsten filaments. A heat source may comprise a plurality of such lamps oriented in a parallel array. However, due to the construction the lamps and the spacing of the lamps from each other within the array, temperature irregularities as great as 40.degree. C. over distances of approximately 1 cm have been observed in the area of illumination of such an array. Such temperature irregularities contribute to lateral temperature gradients which adversely affect the device being manufactured in a manner hereinafter described.
Direct observation of such an array of lamps when energized has indicated that the temperature irregularities stemming from the construction of the lamps are due to a self-shadowing of the coiled tungsten filament, the shadowing of the disk shaped filament holders disposed within the quartz envelope, and the refractive properties of the quartz envelopes. The spacing of the lamps and thus the tungsten filaments within the array also contributes to the non-uniformities in temperature of the radiation emitted by the lamps. Moreover, non-uniformities in reflection of any reflectors employed with the quartz lamps further contributes to the irregularities in temperature experienced by the semiconductor bodies heated by the lamps and thus enhances the creation of lateral temperature gradients. Additionally, any quartz plates disposed between the lamps and the semiconductor bodies such as, for example, a quartz convection suppressor plate or a quartz coverplate for an air cooling channel surrounding the lamps may possess refractive properties which may further contribute to temperature irregularities and the creation of lateral temperature gradients.
During the migration of the molten zone through the semiconductor material, any lateral temperature gradients distort the movement of the molten zone. That is, the geometrical configuration of the regions produced by the movement of the molten zone through the solid body changes in accordance with the thermal gradients, both lateral and normal, in the region of movement. The lateral thermal gradient is of particular concern in that when forming grid structures, for electrical isolation of devices, distortion of the array forming the grid due to these lateral gradients may be so great as to eliminate the outer 2 mm peripheral portion of a wafer from being usable. The geometrical pattern of the grid structure is very distorted. Separation of isolation regions occur and surface tension pulls intersecting regions apart during migration.
It is therefore an object of this invention to provide a new and improved thermal gradient zone melting process which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved thermal gradient zone melting process wherein any lateral and/or radial temperature gradients in a body or wafer of semiconductor material being processed thereby is minimized.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.
In accordance with the teachings of this invention, there is provided a new and improved thermal gradient zone melting method for migrating a molten zone through a solid body of semiconductor material. The method comprises the process steps of selecting a body of single crystal semiconductor material having two major opposed surfaces which are, respectively, the top and bottom surfaces thereof. The body has a predetermined type conductivity, a predetermined level of resistivity, a preferred diamond crystal structure, a preferred planar crystal orientation for at least the top surface, and a first preferred crystal axis and a central axis, which are each substantially perpendicular to the top surface and substantially parallel with each other. A layer of metal of a predetermined thickness and a predetermined geometrical configuration is preferably vapor deposited on the major surface having the preferred planar crystal orientation. The processed body is placed on a support and rotated noncentro-symmetrically about an axis displaced from the central axis of the body while simultaneously translated in a plane parallel to a heat source through a distance heated by the heat source. The heat source comprises a parallel array of lamps. Translation is performed at an angle of from 30.degree. to 60.degree. to the central axis of the parallel array of lamps. Preferably, the lamps are of the infrared type.
The body and the deposited metal are heated to a preselected elevated temperature sufficient to form a melt of a metal-rich semiconductor material on the surface of the body while continuing any one of the aforementioned movements of the body. A temperature gradient is established across the body substantially parallel with the central axis of the body and the first preferred crystal axis of the crystal structure while continuing the movement of the body. The surface on which the melt is formed is retained at the lower temperature. Thereafter, each melt of metal-rich semiconductor material is migrated as a molten zone through the solid body of semiconductor material for a sufficient period of time to reach a predetermined distance into the body from the surface on which the melt is formed. The movement of the body is continued during this migration. A region of recrystallized semiconductor material of the body having solid solubility of the deposited metal therein is formed in the body by each melt. Each region so produced has a predetermined geometric configuration, a substantially uniform cross-sectional area and a substantially uniform level of resistivity throughout the entire region.